Energy in the Tx = 0.5 B^2 Ae. Lg / Uo. Ur, for a ferrite with an air gap most of the energy is stored in the gap, so Ur = 1, and Lg = length of gap
The voltage on the cap is not constant, at fet turn off the current rises in the sec side coil ( how fast it rises depends on a number of factors including the flyback volts on the pri and the leakage inductance pri to sec ) the current is quite high to start with and ramps linearly down wards until the mosfet is turned on again.
So if you can see the cap volts clearly, there will be an ESL/ESR jump as the current jumps up to its max value, on top of this the cap voltage will rise at a falling rate .... - for a large cap this rise may be only a few 10's or 100's of mV.